Method for forming silicon dioxide film on silicon substrate, method for forming oxide film on semiconductor substrate, and method for producing semiconductor device

ABSTRACT

After cleaning a surface of a silicon substrate ( 1 ), impurities and natural oxide film existing on the silicon substrate ( 1 ) are removed by soaking the silicon substrate ( 1 ) in a 0.5%-by-volume HF aqueous solution for 5 minutes. The silicon substrate ( 1 ) is rinsed (cleaned) with ultrapure water for five minutes. Then, the silicon substrate ( 1 ) is soaked for 30 minutes in azeotropic nitric acid heated to an azeotropic temperature of 120.7° C. In this way, an extremely thin chemical oxide film ( 5 ) is formed on the surface of the silicon substrate ( 1 ). Subsequently, a metal film ( 6 ) (aluminum-silicon alloy film) is deposited, followed by heating in a hydrogen-containing gas at 200° C. for 20 minutes. Through the heat processing in the hydrogen-containing gas, hydrogen reacts with interface states and defect states in the chemical oxide film ( 5 ), causing disappearance of the interface states and defect states. As a result, the quality of the film can be improved. Thus, it is possible to form a high quality (with low leak current density), extremely thin silicon dioxide film on the silicon substrate at a low temperature with excellent film-thickness controllability.

TECHNICAL FIELD

In general, the present invention relates to a method of forming asilicon dioxide film on a surface of a silicon substrate, a method offorming an oxide film on a surface of a semiconductor substrate, and amethod of manufacturing a semiconductor device by these methods. Thesemethods can be employed to form an oxide film of an MOS(metal-oxide-semiconductor) device used for semiconductor integratedcircuits, especially to form an extremely thin gate oxide film, acapacitor oxide film, and the like of an MOS transistor, an MOScapacitor, and the like. In particular, the present invention relates toa method of forming a high-quality (with little leak current), extremelythin silicon dioxide film and the like with excellent film-thicknesscontrollability.

The present invention also relates to a method of forming, for example,a silicon dioxide film of a TFT (thin-film transistor) at a lowtemperature.

BACKGROUND ART

Recently, the performance of semiconductor devices, such assemiconductor integrated circuits, has been improving without showingany sign of leveling off, and the need for further integration andhigh-density packaging is becoming more and more compelling. Forexample, one problem of microfabrication for further integration andhigh-density packaging is a problem related to an insulating film, suchas a gate insulating film and a capacitor insulating film, of an MOStransistor, an MOS capacitor, and the like.

In a device (such as a silicon device) formed by using a siliconsubstrate, and in an MOS transistor and an MOS capacitor in particular,a silicon dioxide film is usually used as an insulating film, such as agate insulating film and a capacitor insulating film.

As a result of microfabrication of the device, the insulating filmbecomes extremely thin. For example, if the design rule is not more than0.07 μm, the gate insulating film is required to be not thicker than 1.5nm.

However, considering such problems as the increase of leak current, itis estimated that the limit of microfabrication of silicon dioxide filmis 1.5 nm to 1.2 nm. In view of this, the use of such highly dielectricmaterials as Al₂O₃ and Ta₂O₅ is considered, but by they are not yet usedpractically. Even if these highly dielectric materials are put intopractical use, the introduction of these new materials will require agreat amount of initial investment.

Conventionally, the oxide film (gate oxide film) used as a gateinsulating film of an MOS transistor has been formed by heating asilicon substrate at a high temperature of not lower than 800° C. in anoxidizing gas such as dry oxygen or vapor. However, if an extremely thinoxide film of not thicker than 2 nm is formed by high-temperaturethermal oxidization, there is a problem that the oxide film cannot beused as a gate insulating film because of its high leak current density.In addition, it is difficult to control the thickness of the oxide filmformed by high-temperature thermal oxidization, because the initialgrowth rate of the oxide film is high. Therefore, it is difficult toform an extremely thin oxide film by high-temperature thermaloxidization. Moreover, the high-temperature thermal oxidization also hasa problem that the dopant diffuses when heated at a high temperature,resulting in the destruction of shallow junctions.

Other than high-temperature thermal oxidization, there are such methodsas chemical vapor-phase growth (in which such material as monosilane isthermally decomposed and deposited on a surface of the siliconsubstrate), a method in which an oxide film is formed by anodicoxidation, various kinds of vapor deposition such as sputter vapordeposition, and a method of oxidizing in plasma. However, these methodsalso have the same problems in terms of film quality and film-thicknesscontrollability.

In particular, the increase of the leak current density not only causesvarious problems such as the increase of power consumption of thedevice, the increase of operating temperature, the deterioration ofstability, and the like, but also destabilizes the operation of thedevice if the amount of leak current is substantially equal to theamount of drain current.

In view of these problems, the inventors of the present inventioninvented a method of forming an oxide film by chemical oxidization andfiled a patent application (patent publication 1: Japanese Laid-OpenPatent Publication, Tokukai 2002-64093 (publication date: Feb. 28,2002)). According to this method, a chemical oxide film is formed bysoaking a silicon substrate into, for example, concentrated nitric acid.Then, the chemical oxide film is heat-processed in an inactive gas suchas nitrogen. The heat processing decreases the leak current in the oxidefilm. This method is POA (postoxidization annealing), because the heatprocessing is performed after the oxide film is formed.

The inventers of the present invention invented another method offorming an oxide film by chemical oxidization and filed a patentapplication (patent publication 2: Japanese Laid-Open PatentPublication, Tokukaihei 9-45679 (publication date: Feb. 14, 1997)).According to this method, a chemical oxide film is formed by soaking asilicon substrate into, for example, concentrated nitric acid. Then, ametal film (e.g. platinum) having a function of oxidation catalyst isformed on the oxide film. After that, the oxide film is grown by heatprocessing in an oxidizing atmosphere.

The inventers of the present invention invented yet another method offorming an oxide film by chemical oxidization and filed a patentapplication (patent publication 3: Japanese Laid-Open PatentPublication, Tokukai 2002-57154 (publication date: Feb. 22, 2002)).According to this method, a chemical oxide film is formed by soaking asilicon substrate into, for example, concentrated nitric acid. Then, ametal film (e.g. platinum) having a function of oxidation catalyst isformed on the oxide film, followed by heat processing in an oxidizingatmosphere. Thereafter, the metal film and a part of the oxide film areremoved by etching so as to reduce the thickness of the oxide film, andan electrode is formed on the oxide film.

According to the method of patent publication 1, the heat processing inthe inactive gas is required to be performed at a relatively hightemperature. If the heat processing in the inactive gas is performed ata high temperature, there is a problem that the thickness of the oxidefilm increases due to a very small amount of an oxidizing species, suchas vapor and oxygen, mixed in the inactive gas.

Another problem of the method of patent publication 1 is that diffusionof the dopant is caused by the heat processing at a high temperature asin the case of high-temperature thermal oxidization, resulting in thedestruction of shallow junctions. Moreover, there is also a problem thatthe heat processing at a high temperature cannot decrease the leakcurrent density with sufficient reproducibility.

The method of patent publication 2 is not suitable for forming anextremely thin oxide film, because the method has a step of growing theoxide film. The method of patent publication 2 cannot effectivelydecrease the leak current density of the oxide film, either.

According to the method of patent publication 3, it is difficult tocontrol the thickness of the oxide film, because it is necessary toreduce the thickness by etching. Moreover, if some parts of the oxidefilm are extremely thin as a result of etching, the leak current densityincreases. Therefore, it is difficult to decrease the leak currentdensity with sufficient reproducibility.

Incidentally, the gate oxide film of a TFT has been conventionallyformed by CVD (chemical vapor deposition) in which deposition is causedat a substrate temperature of about 600° C.

In order to manufacture a flexible liquid crystal display, the TFT isrequired to be formed on a substrate of organic material such as PET(polyethylene terephthalate). For this purpose, the TFT must be formedat a low temperature of not higher than 200° C. However, in order todeposit the silicon dioxide film by CVD, it is necessary to heat thesubstrate to a high temperature of 400° C. to 500° C. Thus, thedeposition of silicon dioxide film by CVD is not suitable for theformation of TFT in manufacturing a flexible liquid crystal display.

In the above-described TFT, in general, a relatively high voltage isapplied to the gate electrode. Therefore, in order to prevent dielectricbreakdown, the silicon dioxide film used as the gate oxide film needs tohave a sufficient thickness.

The present invention was made in view of the foregoing problems. Anobject of the present invention is therefore to provide (i) a method offorming an oxide film on a surface of a semiconductor substrate, bywhich a high-quality (with low leak current density), extremely thinoxide film can be formed on a surface of a silicon substrate withexcellent film-thickness controllability and at a low temperature, (ii)in particular, a method of forming a silicon dioxide film on a surfaceof a silicon substrate, by which a high-quality (with low leak currentdensity), extremely thin silicon dioxide film can be formed on a surfaceof a silicon substrate with excellent film-thickness controllability andat a low temperature, and (iii) a method of manufacturing asemiconductor device by these methods. Another object of the presentinvention is to provide a method of forming a silicon dioxide film at alow temperature, so that the silicon dioxide film can be formed on asubstrate of organic material such as PET.

DISCLOSURE OF INVENTION

A method of forming a silicon dioxide film on a surface of a siliconsubstrate in accordance with the present invention includes the stepsof: forming a silicon dioxide film on a surface of a silicon substrateby supplying a drug solution to the surface of the silicon substrate;depositing a film containing metal atoms on the silicon dioxide film;and heat-processing the silicon substrate, on which the film containingmetal atoms is deposited, in a hydrogen-containing gas.

A method of forming a silicon dioxide film on a surface of a siliconsubstrate in accordance with the present invention includes the stepsof: forming a silicon dioxide film on a surface of a silicon substrateby supplying vapor of a drug solution to the surface of the siliconsubstrate; depositing a film containing metal atoms on the silicondioxide film; and heat-processing the silicon substrate, on which thefilm containing metal atoms is deposited, in a hydrogen-containing gas.

According to these methods, through the heat processing in thehydrogen-containing gas, hydrogen reacts with the silicon dioxide film,resulting in disappearance of the interface states and defect states inthe silicon dioxide film. It can be assumed that, at this time, thepresence of the film containing metal atoms facilitates decomposition ofhydrogen and thereby facilitates the disappearance of the interfacestates and defect states in the silicon dioxide film. As a result, it ispossible to improve quality of the silicon dioxide film, and to form ahigh-quality (with low leak current density), extremely thin oxide film.

The thickness of the silicon dioxide film can be controlled easily byadjusting the kind, concentration, and temperature of the drug solution.

According to the foregoing methods, the silicon dioxide film can bemodified through heat processing in the hydrogen-containing gas.Therefore, it is not necessary to perform heat processing in anoxidizing atmosphere. As a result, the thickness of the silicon oxidefilm does not increase easily, that is, the silicon oxide film possessesexcellent film-thickness controllability.

Furthermore, the heat processing can be performed at a relatively lowtemperature. Therefore, the thickness of the silicon dioxide film doesnot increase easily even if oxygen or the like is mixed in, that is, thesilicon dioxide film possesses excellent film-thickness controllability.

Thus, it is possible to form a high-quality (with low leak currentdensity), extremely thin (0.3 nm to 5 nm in thickness) silicon dioxidefilm on the surface of the silicon substrate.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the drug solution is selected from thegroup consisting of nitric acid, sulfuric acid, ozonic water, hydrogenperoxide solution, mixed solution of hydrochloric acid and hydrogenperoxide solution, mixed solution of sulfuric acid and hydrogen peroxidesolution, mixed solution of aqueous ammonia and hydrogen peroxidesolution, mixed solution of sulfuric acid and nitric acid,nitrohydrochloric acid, perchloric acid, and boiling water.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the vapor of a drug solution is vaporof a drug solution selected from the group consisting of nitric acid,sulfuric acid, ozonic water, hydrogen peroxide solution, mixed solutionof hydrochloric acid and hydrogen peroxide solution, mixed solution ofsulfuric acid and hydrogen peroxide solution, mixed solution of aqueousammonia and hydrogen peroxide solution, mixed solution of sulfuric acidand nitric acid, nitrohydrochloric acid, perchloric acid, and water.

By using the foregoing drug solution or the vapor of the foregoing drugsolution, it is possible to form an extremely thin (0.3 nm to 5 nm inthickness) silicon dioxide film at a low temperature of not higher than500° C. with excellent film-thickness controllability.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the drug solution is selected from thegroup consisting of azeotropic nitric acid that is an azeotropic mixtureof nitric acid and water, azeotropic sulfuric acid that is an azeotropicmixture of sulfuric acid and water, and azeotropic perchloric acid thatis an azeotropic mixture of perchloric acid and water.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the vapor of a drug solution is vaporof a drug solution selected from the group consisting of azeotropicnitric acid that is an azeotropic mixture of nitric acid and water,azeotropic sulfuric acid that is an azeotropic mixture of sulfuric acidand water, and azeotropic perchloric acid that is an azeotropic mixtureof perchloric acid and water.

In an azeotropic state, the concentration of the drug solution or of thevapor of the drug solution does not easily change with time. Therefore,the thickness of the silicon dioxide film to be formed can be controlledwith excellent reproducibility by using the drug solution or of thevapor of the drug solution in an azeotropic state.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis any one of the foregoing methods, wherein: the film containing metalatoms is a film containing metal atoms selected from the groupconsisting of aluminum, magnesium, nickel, chrome, platinum, palladium,tungsten, titanium, and tantalum.

If the film containing metal atoms is deposited on the silicon dioxidefilm, the heat processing in the hydrogen-containing gas can easilycause reaction of hydrogen with the interface states and defect states.As a result, the interface states and defect states of the silicondioxide film disappear effectively, thereby improving the quality of thesilicon dioxide film.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis any one of the foregoing methods, wherein: the hydrogen-containinggas is hydrogen or a mixed gas of hydrogen and a gas selected from thegroup consisting of nitrogen, argon, neon, water vapor, and oxygen.

If the foregoing gas is used, hydrogen reacts with the interface statesand defect states of the silicon dioxide film. As a result, theinterface states and defect states of the silicon dioxide filmdisappear, thereby improving the quality of the silicon dioxide film.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis any one of the foregoing methods, wherein: a temperature of thesilicon substrate during the heat processing in the hydrogen-containinggas is within a range of 50° C. to 350° C.

Within the foregoing temperature range, the interface states and defectstates in the silicon dioxide film disappear effectively, and reactionbetween (i) the film containing metal atoms and (ii) the silicon dioxidefilm can be prevented.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the heat processing in thehydrogen-containing gas is conducted for a period within a range of 1minute to 120 minutes.

If the period of the heat processing in the hydrogen-containing gas isnot shorter than 1 minute, the hydrogen diffuses (i) the film containingmetal atoms and (ii) the silicon dioxide film, thereby modifying thesilicon dioxide film effectively. In addition, if the time of the heatprocessing in the hydrogen-containing gas is not longer than 120minutes, the increase of device preparation time is unlikely to be aproblem. Moreover, even if the temperature of the heat processing in thehydrogen-containing gas is close to 350° C., reaction between (i) thefilm containing metal atoms and (ii) the silicon dioxide film can beprevented as long as the period of heat processing is not longer than120 minutes.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the foregoing methodis any one of the foregoing methods, wherein: a natural oxide film orimpurities existing on the surface of the silicon substrate are removedbefore forming the silicon dioxide film on the surface of the siliconsubstrate.

If, as described above, the natural oxide film or impurities existing onthe surface of the silicon substrate are removed and the clean siliconsurface is exposed, it is possible to form a high-quality silicondioxide film.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the foregoing methodis the foregoing method, wherein: the silicon substrate is heated insupplying the vapor of a drug solution to the surface of the siliconsubstrate. By doing so, it is possible to increase the rate ofoxidization.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: a temperature of the silicon substrateheated in supplying the vapor of a drug solution to the surface of thesilicon substrate is within a range of 50° C. to 500° C.

By setting the temperature of the silicon substrate to not lower than50° C. in heating the silicon substrate in supplying the vapor of a drugsolution, it is possible to increase the rate of oxidizationeffectively. In this way, a silicon dioxide film having a thickness ofnot less than 1 nm can be formed easily (if the temperature is lowerthan 50° C., it is difficult to form a silicon dioxide film having athickness of not less than 1 nm). Moreover, by setting the temperatureto not higher than 500° C., it is possible to avoid excessivelyaccelerating oxidization, and thereby control the film thickness easily.

A method of forming a silicon dioxide film on a surface of a siliconsubstrate in accordance with the present invention includes the stepsof: forming a silicon dioxide film on a surface of a silicon substrateby supplying a drug solution to the surface of the silicon substrate;and heat-processing the silicon substrate, on which the silicon dioxidefilm is formed, in a hydrogen-containing gas.

A method of forming a silicon dioxide film on a surface of a siliconsubstrate in accordance with the present invention includes the stepsof: forming a silicon dioxide film on a surface of a silicon substrateby supplying vapor of a drug solution to the surface of the siliconsubstrate; and heat-processing the silicon substrate, on which thesilicon dioxide film is formed, in a hydrogen-containing gas.

According to the foregoing method, through the heat processing in hehydrogen-containing gas, the hydrogen reacts with the interface statesand defect states in the silicon dioxide film, thereby forming Si—Hbonds. As a result, the interface states and defect states disappear. Inthis case, unlike the foregoing method, the effect of decomposinghydrogen due to the presence of the film containing metal atoms cannotbe attained. However, by increasing the temperature of the heatprocessing, it is possible to accelerate reaction of the hydrogen withthe interface states and defect states. As a result, quality of thesilicon dioxide film is improved, and it is possible to form ahigh-quality (with low leak current density), extremely thin oxide film.

In the foregoing method, it is preferable that a temperature of thesilicon substrate during the heat processing in the hydrogen-containinggas is within a range of 350° C. to 500° C.

If the temperature of the silicon substrate during the heat processingin the hydrogen-containing gas is lower than 350° C., it is difficult toform the Si—H bonds by reacting the hydrogen with the interface statesand defect states. If, on the other hand, the temperature is higher than500° C., the Si—H bonds split up even if they are successfully formed,and the interface states and defect states are generated again.Therefore, in order to cause disappearance of the interface states anddefect states effectively, it is preferable to set the temperature to350° C. to 500° C.

A method of forming a silicon dioxide film on a surface of a siliconsubstrate in accordance with the present invention includes the stepsof: forming a silicon dioxide film on a surface of a silicon substrateby supplying vapor of a drug solution to the surface of the siliconsubstrate; and heat-processing the silicon substrate, on which thesilicon dioxide film is formed, in a hydrogen-containing gas.

In order to form the silicon dioxide film through deposition by CVD, thesubstrate must be heated to a high temperature of about 400° C. to 500°C. Therefore, forming the silicon dioxide film by CVD is not suitable,for example, for forming TFTs in manufacturing a flexible liquid crystaldisplay.

In contrast, according to the foregoing method, the silicon dioxide filmcan be formed easily even if the silicon substrate is at a lowtemperature of not higher than 200° C. Therefore, this method issuitable for forming TFTs in manufacturing a flexible liquid crystaldisplay.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the vapor of a drug solution is vaporof a drug solution selected from the group consisting of nitric acid,sulfuric acid, and perchloric acid.

According to the foregoing method, even if the silicon substrate is at alow temperature of not higher than 200° C., it is easy to form achemical oxide film having a thickness of not less than 2 nm, and it iseven possible to form a chemical oxide film having a thickness of notless than 10 nm.

It is preferable that a method of forming a silicon dioxide film on asurface of a silicon substrate in accordance with the present inventionis the foregoing method, wherein: the vapor of a drug solution is vaporof a drag solution selected from the group consisting of azeotropicnitric acid that is an azeotropic mixture of nitric acid and water,azeotropic sulfuric acid that is an azeotropic mixture of sulfuric acidand water, and azeotropic perchloric acid that is an azeotropic mixtureof perchloric acid and water.

In an azeotropic state, the concentration of the drug solution or of thevapor of the drug solution does not easily change with time. Therefore,by using the drug solution or the vapor of the drug solution in theazeotropic state, the thickness of the silicon dioxide film to be formedcan be controlled with excellent reproducibility.

A method of manufacturing a semiconductor device in accordance with thepresent invention includes the step of: forming a silicon dioxide filmon a surface of a silicon substrate by any one of the foregoing methods.According to this method, it is possible to manufacture a semiconductordevice that includes the high-quality, extremely thin oxide film.

The present invention is also applicable in the case of forming an oxidefilm on a surface of a semiconductor substrate other than the siliconsubstrate. Specifically, a method of forming an oxide film on a surfaceof a semiconductor substrate in accordance with the present inventionincludes the steps of: forming an oxide film on a surface of asemiconductor substrate by supplying a drug solution to the surface ofthe semiconductor substrate; depositing a film containing metal atoms onthe oxide film; and heat-processing the semiconductor substrate, onwhich the film containing metal atoms is deposited, in ahydrogen-containing gas.

A method of forming an oxide film on a surface of a semiconductorsubstrate in accordance with the present invention includes the stepsof: forming an oxide film on a surface of a semiconductor substrate bysupplying vapor of a drug solution to the surface of the semiconductorsubstrate; depositing a film containing metal atoms on the oxide film;and heat-processing the semiconductor substrate, on which the filmcontaining metal atoms is deposited, in a hydrogen-containing gas.

Like the foregoing methods, these methods make it possible to form ahigh-quality (with low leak current density), extremely thin oxide filmon the surface of the semiconductor substrate.

A method of manufacturing a semiconductor device in accordance with thepresent invention includes the step of: forming an oxide film on asurface of a semiconductor substrate by the foregoing method. Accordingto this method, it is possible to manufacture a semiconductor devicethat includes the high-quality, extremely thin oxide film.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) to 1(f) are cross-sectional views respectively illustratingsteps in a method of forming a silicon dioxide film on a surface of asilicon substrate in accordance with a first embodiment of the presentinvention.

FIG. 2 is a graph illustrating the relationship between applied voltageand leak current density, with respect to a silicon dioxide film (a)(formed for the purpose of comparison) and a silicon dioxide film (b)(formed by the method of FIGS. 1(a) to 1(f)).

FIG. 3 is a graph illustrating the relationship between applied voltageand electric capacitance, with respect to a silicon dioxide film (a)(formed for the purpose of comparison) and a silicon dioxide film (b)(formed by the method of FIGS. 1(a) to 1(f)).

FIG. 4 is a graph illustrating changes of the leak current density ofthe silicon dioxide film formed at different temperatures of heatprocessing in a hydrogen-containing gas by the method of FIGS. 1(a) to1(f).

FIG. 5 is a graph illustrating X-ray photoelectron spectrums from the 2porbit of silicon atoms in a silicon dioxide film (a) (formed for thepurpose of comparison), a silicon dioxide film (b) (formed by the methodof FIGS. 1(a) to 1(f)), and a silicon dioxide film (c) (formed by themethod of FIGS. 1(a) to 1(f) at a different temperature of the heatprocessing in the hydrogen-containing gas).

FIG. 6 is a graph illustrating valence band spectrums of a clean siliconsubstrate (a), a silicon substrate (b) (on which a silicon dioxide filmis formed for the purpose of comparison), and a silicon substrate (c)(on which a silicon dioxide film is formed by the method of FIGS. 1(a)to 1(f)), measured by X-ray photoelectron method.

FIG. 7 is a band diagram illustrating an expected band state of asilicon dioxide film formed without performing the heat processing inthe hydrogen-containing gas by the method of FIGS. 1(a) to 1(f) and of asilicon dioxide film formed by performing the heat processing.

FIGS. 8(a) to 8(e) are cross-sectional views respectively illustratingsteps in a method of forming a silicon dioxide film on a surface of asilicon substrate in accordance with a second embodiment of the presentinvention.

FIG. 9 is a graph illustrating the relationship between applied voltageand leak current density, with respect to a silicon dioxide film (a)(formed for the purpose of comparison) and a silicon dioxide film (b)(formed by the method of FIGS. 8(a) to 8(e)).

FIG. 10 is a graph illustrating the relationship between applied voltageand electric capacitance, with respect to a silicon dioxide film formedby the method of FIGS. 8(a) to 8(e).

FIG. 11 is a graph illustrating changes of the leak current density ofthe silicon dioxide film formed at different temperatures of heatprocessing in a hydrogen-containing gas by the method of FIGS. 8(a) to8(e).

FIG. 12 is a graph illustrating the relationship between applied voltageand electric capacitance, with respect to a silicon dioxide film formedby the method of FIGS. 8(a) to 8(e) except that the temperature of theheat processing in the hydrogen-containing gas is changed to 600° C.

FIG. 13 is a graph illustrating X-ray photoelectron spectrums from the2p orbit of silicon atoms in a silicon dioxide film (a) (formed for thepurpose of comparison) and a silicon dioxide film (b) (formed by themethod of FIGS. 8(a) to 8(e)).

FIG. 14 is a graph illustrating an X-ray photoelectron spectrum from the2p orbit of silicon atoms in a silicon dioxide film formed by a thirdembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

The following describes a first embodiment of the present invention,with reference to FIG. 1(a) to FIG. 7.

Described first with reference to FIGS. 1(a) to 1(f) is a method offorming a silicon dioxide film on a silicon substrate in accordance withthe present embodiment. In the present embodiment, a step of forming anMOS capacitor by using a silicon substrate is described. Although thepresent embodiment assumes a case where the silicon dioxide film isformed on a silicon substrate 1, the present invention is alsoapplicable, for example, to a case where the silicon dioxide film isformed on a ball-shaped silicon bulk or on a silicon film formed byepitaxial growth. In this specification, the member (e.g. siliconsubstrate 1) used as a base of the, silicon dioxide film to be formed isreferred to as “silicon substrate”.

First, an isolation region 2 and an active region 4 were formed on thesilicon substrate 1. On a surface of the active region 4, there was anatural oxide film 3 (FIG. 1(a)). In the present embodiment, the siliconsubstrate 1 was a p-type (100) substrate whose specific resistance was10Ωcm to 15Ωcm. After boron was injected as a channel stopper into thesubstrate 1, a LOCOS (local oxidation of silicon) oxide film having athickness of 500 nm was formed as the isolation region 2.

Next, in order to clean the surface of the active region 4, the wafer(the term “wafer” is used when reference is made to the entire bodyincluding the silicon substrate 1 and the oxide film and the like formedon the silicon substrate 1) was cleaned by known RCA cleaning method (W.Kern and D. A. Poutinen, RCA Review 31, p. 187, 1970). After that, thecleaned wafer was soaked in a 0.5%-by-volume dilute hydrofluoric acidsolution for five minutes, so as to remove the natural oxide film 3 fromthe surface of the active region 4 (FIG. 1(b)). In order to form ahigh-quality, extremely thin silicon dioxide film on the surface of thesilicon substrate 1, it is preferable that a clean silicon surface isexposed. It is therefore important to completely remove the naturaloxide film 3 from the surface of the silicon substrate 1 and to removeimpurities from the surface of the silicon substrate 1.

Next, the wafer was rinsed (cleaned) with ultrapure water for fiveminutes, and then soaked in a highly oxidative drug solution (oxidizingdrug solution). In the present embodiment, the wafer was soaked for 30minutes in azeotropic nitric acid (68% by weight) whose boilingtemperature is 120.7° C. In this way, a chemical oxide film (silicondioxide film) 5 having a thickness of 1.4 nm was formed on the surfaceof the silicon substrate 1 in the active region 4 (FIG. 1(c)).

The drug solution in which the wafer is soaked in forming the chemicaloxide film 5 can be a drug solution selected from the group consistingof nitric acid, sulfuric acid, ozonic water (water in which severaldozen ppm of ozone is dissolved), hydrogen peroxide solution, mixedsolution of hydrochloric acid and hydrogen peroxide solution, mixedsolution of sulfuric acid and hydrogen peroxide solution, mixed solutionof aqueous ammonia and hydrogen peroxide solution, mixed solution ofsulfuric acid and nitric acid, nitrohydrochloric acid, perchloric acid,and boiling water. The terms “nitric acid” and “sulfuric acid” used hereincludes solutions of these substances.

By using these drug solutions, an extremely thin chemical oxide film 5having a thickness of, for example, 0.3 nm to 5 nm can be formed at alow temperature of not higher than 500° C. with excellent film-thicknesscontrollability.

Therefore, as a method of forming the chemical oxide film 5 on thesurface of the silicon substrate 1, the following methods can beemployed: a method of soaking the wafer in nitric acid, a method ofsoaking the wafer in sulfuric acid, a method of soaking the wafer inozonic water, a method of soaking the wafer in hydrogen peroxidesolution, a method of soaking the wafer in mixed solution ofhydrochloric acid and hydrogen peroxide solution, a method of soakingthe wafer in sulfuric acid and hydrogen peroxide solution, a method ofsoaking the wafer in aqueous ammonia and hydrogen peroxide solution, amethod of soaking the wafer in mixed solution of sulfuric acid andnitric acid, a method of soaking the wafer in nitrohydrochloric acid, amethod of soaking the wafer in perchloric acid, and a method of soakingthe wafer in boiling water.

At this time, the thickness of the chemical oxide film 5 can becontrolled easily by adjusting the kind, concentration, and temperatureof the drug solution.

It is not always necessary to soak the wafer into the drug solution, aslong as the drug solution can be supplied to the surface of the wafer.

The chemical oxide film 5 can also be formed by a method of exposing thewafer to vapor of drug solution, instead of the method of soaking thewafer in the drug solution. The vapor of drug solution to which thewafer is exposed in forming the chemical oxide film 5 can be the vaporof the drug solution described above as a drug solution into which thewafer is soaked in forming the chemical oxide film 5, that is, vapor ofa drug solution selected from the group consisting of nitric acid,sulfuric acid, ozonic water (water in which several dozen ppm of ozoneis dissolved), hydrogen peroxide solution, mixed solution ofhydrochloric acid and hydrogen peroxide solution, mixed solution ofsulfuric acid and hydrogen peroxide solution, mixed solution of aqueousammonia and hydrogen peroxide solution, mixed solution of sulfuric acidand nitric acid, nitrohydrochloric acid, perchloric acid, and water.

In exposing the wafer to the vapor of the drug solution, it ispreferable to heat the wafer. By heating the wafer, it is possible toincrease the rate of oxidization.

It is preferable that the temperature of the wafer at this time iswithin the range of 50° C. to 500° C. By setting the temperature of thewafer not lower than 50° C., it is possible to increase the rate ofoxidization effectively. In this way, a chemical oxide film 5 having athickness of not less than 1 nm can be formed easily (if the temperatureof the wafer is lower than 50° C., it is difficult to form a chemicaloxide film 5 having a thickness of not less than 1 nm). Moreover, bysetting the temperature of the wafer not higher than 500° C., it ispossible to avoid excessively accelerating oxidization, and therebycontrol the thickness of the chemical oxide film 5 easily.

It is preferable that the drug solution into which the wafer is soakedor the vapor of drug solution to which the wafer is exposed in formingthe chemical oxide film 5 is a drug solution or vapor of a drug solutionselected from the group consisting of azeotropic nitric acid that is anazeotropic mixture of nitric acid and water, azeotropic sulfuric acidthat is an azeotropic mixture of sulfuric acid and water, and azeotropicperchloric acid that is an azeotropic mixture of perchloric acid andwater. In an azeotropic state, the concentration of the drug solution orvapor does not easily change with time. Therefore, the thickness of thechemical oxide film 5 to be formed can be controlled with excellentreproducibility. This is why the foregoing drug solutions and vapor arepreferable.

In the present embodiment, azeotropic nitric acid is used in order toform a clean and high-quality chemical oxide film 5 including no suchsubstance as heavy metal. Although the thickness of the chemical oxidefilm 5 is 1.4 nm in the present embodiment, an extremely thin chemicaloxide film 5 having a thickness of, for example, 0.3 nm to 5 nm(preferably 0.5 nm to 2.0 nm) may be formed.

Next, a metal film 6 (film containing metal atoms) was deposited on thechemical oxide film 5 and the isolation region 2 (FIG. 1(d)). The metalfilm 6 was an aluminum (aluminum-silicon alloy) film having a thicknessof 200 nm and including 1% by weight of silicon. The metal film 6 wasdeposited by resistive heating vapor deposition. Examples of the filmcontaining metal atoms are a film containing metal atoms selected fromthe group consisting of aluminum, magnesium, nickel, chrome, platinum,palladium, tungsten, titanium, and tantalum. It is preferable that thefilm containing metal atoms is a film containing active metal atoms. Forexample, it is preferable that the film containing metal atoms is ametal film of aluminum, magnesium, nickel, or the like, or an alloy filmsuch as silicon-containing aluminum. Alternatively, the film containingmetal atoms may be a compound such as titanium nitride and tantalumpentoxide.

Next, the wafer is heated in a hydrogen-containing gas in an electricfurnace. In the present embodiment, the wafer was heated in a mixed gasof hydrogen and nitrogen (nitrogen containing 5% of hydrogen) at 200° C.for 20 minutes. This heat processing, being performed after theformation of the metal film 6, is PMA (post metallization annealing).This heat processing is performed to react hydrogen with interfacestates and defect states and thereby cause them to disappear. In thisway, the electric property of the chemical oxide film 5 is improved. Itcan be assumed that the presence of the metal film 6 makes it easier todecompose the hydrogen, and thereby makes it easy to cause disappearanceof the interface states and defect states in the chemical oxide film 5.The heat processing did not change the thickness of the chemical oxidefilm 5. As a result of the heat processing, a modified chemical oxidefilm 7 (hereinafter referred to as “modified oxide film” or “modifiedsilicon dioxide film”) was formed (FIG. 1(e)).

Examples of the hydrogen-containing gas used in the heat processing arehydrogen or a mixed gas of hydrogen and a gas selected from the groupconsisting of nitrogen, argon, neon, water vapor, and oxygen.

Therefore, other than in the mixed gas of hydrogen and nitrogen, theheat processing can also be performed, for example, in hydrogen, in amixed gas of hydrogen and nitrogen, in a mixed gas of hydrogen andargon, in a mixed gas of hydrogen and neon, in a mixed gas of hydrogenand water vapor, and in a mixed gas of hydrogen and oxygen.

The temperature of the wafer in the heat processing is not limited to200° C. Substantially the same effect of improving the electric propertyof the chemical oxide film 5 is attained as long as the temperature ofthe wafer is within the range of 50° C. to 350° C. The period of heatprocessing is not limited to 20 minutes. Substantially the same effectof improving the electric property of the chemical oxide film 5 isattained as long as the period of heat processing is 1 minute to 120minutes. If the period of heat processing is not shorter than 1 minute,the hydrogen diffuses the metal film 6 and the chemical oxide film 5,thereby modifying the chemical oxide film 5 effectively. In addition, ifthe period of heat processing is not longer than 120 minutes, theincrease of device preparation time is unlikely to be a problem.Moreover, even if the temperature of the wafer in the heat processing isclose to 350° C., reaction between the metal film 6 and the chemicaloxide film 5 can be prevented as long as the period of heat processingis not longer than 120 minutes.

Thus, according to the present method, the chemical oxide film 5 can bemodified through the heat processing in the hydrogen-containing gas.Therefore, it is not necessary to perform heat processing in anoxidizing atmosphere. As a result, the thickness of the chemical oxidefilm does not increase easily, that is, excellent film-thicknesscontrollability is attained.

Furthermore, the heat processing in the hydrogen-containing gas iseffective even at a relatively low temperature. Therefore, the thicknessof the chemical oxide film does not increase easily even if oxygen orthe like is mixed in, that is, excellent film-thickness controllabilityis attained.

In order to increase the film thickness intentionally, thehydrogen-containing gas may be a mixed gas of hydrogen and water vapor,a mixed solution of hydrogen and oxygen, or the like.

Then, by a known photography technology, a patterned resist film (notshown) was formed on the metal film 6. Subsequently, by a known dryetching technology, the metal film 6 was etched and patterned. In thisway, an electrode 8 was formed (FIG. 1(f)).

Described next are properties of the chemical oxide film 5 and themodified chemical oxide film 7 formed by the foregoing method. Unlessotherwise noted, in the present embodiment, the term “chemical oxidefilm formation processing” refers to the processing for forming thechemical oxide film 5 by soaking the wafer (finished with suchprocessing as removal of natural oxide film and cleaning) for 30 minutesin 68%-by-weight azeotropic nitric acid heated to the azeotropictemperature of 120.7° C. (processing of FIGS. 1(a) to 1(c)); the term“metal film formation processing” refers to the processing for formingthe metal film 6 (which is made of aluminum-silicon alloy) having athickness of 200 nm on the chemical oxide film 5 formed by the chemicaloxide film formation processing (processing of FIG. 1(d)); and the term“oxide film modifying heat processing” refers to the processing ofintroducing the wafer after the metal film formation processing into anelectric furnace and heating the wafer in nitrogen (which contains 5% ofhydrogen) at 200° C. for 20 minutes (processing of FIG. 1(e)).

FIG. 2 is a graph showing the relationship between the voltage appliedto the electrode (silicon substrate reverse side electrode) 8, which isprovided on the reverse side of the silicon substrate 1, and the leakcurrent density (measurement result) of the leak current flowing in thechemical oxide film 5 or the modified chemical oxide film 7. In FIG. 2,plot (a) indicates the leak current density of the chemical oxide film 5formed by the chemical oxide film formation processing and metal filmformation processing, and plot (b) indicates the leak current density ofthe modified chemical oxide film 7 formed by the chemical oxide filmformation processing, metal film formation processing, and oxide filmmodifying heat processing.

From plot (a), it was found that the leak current density was relativelyhigh if the chemical oxide film 5 was not subjected to the oxide filmmodifying heat processing. If the applied voltage was 1V, the leakcurrent density was about 2 A/cm², which was nearly equal to that of theoxide film having a thickness of 1.4 nm formed by ordinaryhigh-temperature thermal oxidization. On the other hand, from plot (b),it was found that the leak current density was decreased to aboutone-fifth of the leak current density of plot (a) if the chemical oxidefilm 5 was subjected to the oxide film modifying heat processing. If theapplied voltage was 1V, the leak current density was about 0.4 A/cm²,which was lower than that of the oxide film formed by high-temperaturethermal oxidization.

FIG. 3 is a graph (C-V curve) illustrating the relationship between theapplied voltage and the electric capacitance that depends upon thechemical oxide film 5 or the modified chemical oxide film 7. In FIG. 3,curve (a) is a C-V curve on the chemical oxide film 5 formed by thechemical oxide film formation processing and metal film formationprocessing, and curve (b) is a C-V curve on the modified chemical oxidefilm 7 formed by the chemical oxide film formation processing, metalfilm formation processing, and oxide film modifying heat processing.

The curve (a) had a shoulder A. This indicates the existence of defectstates in the chemical oxide film 5 and the existence of interfacestates at an interface of oxide film/silicon. On the other hand, thecurve (b) had no shoulder. This indicates that the defect states andinterface states disappeared as a result of the oxide film modifyingheat processing.

FIG. 4 is a graph illustrating changes of the leak current density ofthe modified chemical oxide film 7 formed by the chemical oxide filmformation processing, metal film formation processing, and oxide filmmodifying heat processing, the changes being caused by changing thetemperature of the wafer in the oxide film modifying heat processing.The result of FIG. 4 is about the leak current density of the case wherethe applied voltage is 1V, and indicates relative values of the leakcurrent density (relative leak current density) on the assumption thatthe leak current density is 1 if the oxide film modifying heatprocessing is not performed.

From FIG. 4, it was found that the leak current density decreased notonly when the temperature of the wafer in the oxide film modifying heatprocessing was 200° C., but also when the temperature of the wafer waswithin the range of 50° C. to 350° C. It was also found that, if thetemperature of the wafer was within the range of 100° C. to 250° C., theelectric property of the modified chemical oxide film 7 improveddrastically, because the leak current density decreased to not higherthan 50% of the case where the oxide film modifying heat processing wasnot performed.

FIG. 5 is a graph illustrating X-ray photoelectron spectrums from the 2porbit of silicon atoms of the chemical oxide film formed on the siliconsubstrate 1. In FIG. 5, spectrum (a) is an X-ray photoelectron spectrumobserved after the chemical oxide film 5 was formed by the chemicaloxide film formation processing; spectrum (b) is an X-ray photoelectronspectrum observed after (i) the modified chemical oxide film 7 wasformed by the chemical oxide film formation processing, metal filmformation processing, and oxide film modifying heat processing, and (ii)the aluminum-silicon alloy film was removed by etching with hydrochloricacid; and spectrum (c) is an X-ray photoelectron spectrum observed after(i) the modified chemical oxide film 7 was formed by the chemical oxidefilm formation processing, metal film formation processing, and oxidefilm modifying heat processing in which the wafer was heated to a newlyset temperature of 400° C., and (ii) the aluminum-silicon alloy film wasremoved by etching with hydrochloric acid.

The X-ray photoelectron spectrums were measured by using ESCALAB220i-XL(product of VG). The X-ray source was Al Ka radiation, which has energyof 1487 eV. The photoelectrons were observed in a directionperpendicular to the surface.

The peak (1) in FIG. 5 is caused by the photoelectrons from the 2p orbitof silicon atoms of the silicon substrate 1. The peak (2) in FIG. 5 iscaused by photoelectrons from the 2p orbit of the silicon atoms of thechemical oxide film 5 or the modified chemical oxide film 7. Based onthe ratio of the area intensity at peak (2) with respect to the areaintensity at peak (1), the thickness of the chemical oxide film 5 andthe modified chemical oxide film 7 was calculated. Here, an average freepath of the photoelectrons from the 2p orbit of silicon atoms was 3.3 nmin the chemical oxide film 5 and in the modified chemical oxide film 7,and 2.7 nm in the silicon substrate 1. The average free path wasdetermined so that the thickness (not less than 3 nm) of the oxide filmrecorded the same value when measured by ellipsometry and whencalculated based on the X-ray photoelectron spectrum.

Based on the ratio of the area intensity at peak (2) of spectrum (a)with respect to the area intensity at peak (1) of spectrum (a), thethickness of the chemical oxide film 5 was calculated as 1.4 nm.

In spectrum (b), the ratio of the area intensity at peak (2) withrespect to the area intensity at peak (1) was almost identical to thatof the spectrum (a). This means that the thickness of the modifiedchemical oxide film 7 hardly changes through the oxide film modifyingheat processing in which the temperature of the wafer is 200° C.

In spectrum (c), the ratio of the area intensity at peak (2) withrespect to the area intensity at peak (1) decreases drastically. Basedon the ratio of the area intensity, the thickness of the chemical oxidefilm 7 in this case was calculated as 0.2 nm, which indicates a decreaseof the film thickness.

This experimental result shows that, if the wafer is heated to atemperature as high as 400° C. in the oxide film modifying heatprocessing, the chemical oxide film reacts with the aluminum-siliconalloy film and turns into another substance, thereby reducing thethickness of the chemical oxide film. Thus, it is clearly preferable toset the temperature of the wafer not higher than 350° C. in the oxidefilm modifying heat processing. The reaction between the chemical oxidefilm and the aluminum-silicon alloy might involve reaction betweenaluminum and oxygen atoms in the chemical oxide film, thereby producingalumina.

FIG. 6 is a graph illustrating valence band spectrums observed by X-rayphotoelectric method. In FIG. 6, spectrum (a) is a spectrum of a cleansilicon surface; spectrum (b) is a valence band spectrum observed afterthe chemical oxide film 5 was formed by the chemical oxide filmformation processing; and spectrum (c) is a valence band spectrumobserved after (i) the modified chemical oxide film 7 was formed by thechemical oxide film formation processing, metal film formationprocessing, and oxide film modifying heat processing, and (ii) thealuminum-silicon alloy film was removed by etching with hydrochloricacid. In order to show a valence band spectrum of only the chemicaloxide film 5 or the modified chemical oxide film 7, spectrums (b) and(c) are shown as a result of subtraction of the spectrum of the cleansilicon surface (spectrum (a)) from the spectrum of the chemical oxidefilm 5 or the modified chemical oxide film 7 on the silicon substrate 1.

From spectrum (b), it was found that the end of the valence band of thechemical oxide film 5 was at an energy position lower than that of theend of the silicon valence band by 3.8 eV. It was also found that thereis a shoulder peak A in the vicinity of the end of the valence band ofthe chemical oxide film 5. It can be assumed that this shoulder peak Ais caused by impurities such as OH. Therefore, the shoulder peak Aindicates that the impurities have energy level in the vicinity of theend of the valence band in the band gap of the chemical oxide film 5.

From spectrum (c), it was found that the end of the valence band of thechemical oxide film, having been shifted to the low-energy side by about0.5 eV through the oxide film modifying heat processing, was at anenergy position lower than that of the end of the silicon valence bandby 4.3 eV.

This experimental result shows that the band gap energy of the chemicaloxide film increased through the oxide film modifying heat processing.Since there was no shoulder peak in the vicinity of the end of thevalence band of the modified chemical oxide film 7, it was also foundthat the energy level in the band gap of the chemical oxide filmdisappeared through the oxide film modifying heat processing.

FIG. 7 is a band diagram illustrating an expected band state of thechemical oxide film. For the sake of simplicity, this band diagram isillustrated in a flat band state. If the oxide film modifying heatprocessing was not performed after the chemical oxide film 5 is formed,the valence band of the chemical oxide film 5 was at an energy positionA, which was lower than that of the end of the silicon valence band by3.8 eV. In the vicinity of the end A of the valence band of the chemicaloxide film 5, there was energy state A′ having a low state density.

On the other hand, if the metal film formation processing and oxide filmmodifying heat processing were performed after the formation of thechemical oxide film 5 in forming the modified chemical oxide film 7, theenergy state A′ disappeared. In addition, the end of the valence band ofthe modified chemical oxide film 7 shifted by 0.5 eV to the low-energyside of the end of the valence band of the chemical oxide film 5,thereby moving to an energy position B, which was lower than the end ofthe silicon valence band by 4.3 eV.

As described above, if the chemical oxide film 5 is formed by thechemical oxide film formation processing and metal film formationprocessing, and modified to the modified chemical oxide film 7 by theoxide film modifying heat processing, the leak current density of thechemical oxide film can be decreased. Although the cause of thisdecrease is still unknown at this time, the following describes what theinventors of the present invention consider as the most rationalaccount.

It can be assumed that, if an oxide film is formed by oxidizing siliconat a low temperature with such substance as highly oxidative nitricacid, the stress at the interface is lower than in the case wherethermal oxidation is performed at a high temperature. However, in achemical oxide film, there are silicon dangling bond interface statesdue to unreacted silicon, and defect states due to impurities such asOH. Therefore, leak current flows through the interface states and/ordefect states.

If the heat processing is performed in a hydrogen-containing gas,hydrogen is isolated on the surface of the metal film, and atomichydrogen is injected into the chemical oxide film. The injected atomichydrogen reacts with the interface states and/or defect states andthereby removes them. As a result, being unable to flow through theinterface states and/or defect states, the leak current can flow only bythe quantum mechanical tunnel mechanism. Moreover, since the heatprocessing in a hydrogen-containing gas increases the band gap energy ofthe chemical oxide film, the tunnel current that flows by the quantummechanical tunnel mechanism also decreases. It can be assumed that theleak current density decreases as a result.

The extremely thin silicon dioxide film formed by the method of thepresent embodiment as described above can be used not only as anextremely thin gate oxide film of MOS transistors and MOS capacitors,but also for various other purposes. For example, it is possible toadopt the method of forming a silicon dioxide film in accordance withthe present embodiment as a step of forming a silicon dioxide film onthe surface of a silicon substrate in a method of manufacturing asemiconductor device. In this case, the metal film 6 may be patternedand used as a conductive layer of the electrode 8 or the like of thesemiconductor device. Alternatively, the metal layer 6 may be removedcompletely, and the conductive layer may be formed separately.

In the present embodiment, the silicon dioxide film is formed on thesilicon substrate. However, the present invention is also applicable tothe case where the oxide film is formed on the surface of asemiconductor substrate other than the silicon substrate. Examples ofthe semiconductor substrate are silicon carbide (SiC) and silicongermanium (SiGe).

Specifically, a method of forming an oxide film on a surface of asemiconductor substrate in accordance with the present inventionincludes the steps of: forming an oxide film on a surface of asemiconductor substrate by supplying a drug solution to the surface ofthe semiconductor substrate; depositing a film containing metal atoms onthe oxide film; and heat-processing the semiconductor substrate, onwhich the film containing metal atoms is deposited, in ahydrogen-containing gas.

A method of forming an oxide film on a surface of a semiconductorsubstrate in accordance with the present invention includes the stepsof: forming an oxide film on a surface of a semiconductor substrate bysupplying vapor of a drug solution to the surface of the semiconductorsubstrate; depositing a film containing metal atoms on the oxide film;and heat-processing the semiconductor substrate, on which the filmcontaining metal atoms is deposited, in a hydrogen-containing gas.

By these methods, a high-quality (with low leak current density),extremely thin oxide film can be formed on the surface of asemiconductor substrate. The method of forming an oxide film on asurface of a semiconductor substrate can be adopted as a step of formingan oxide film on a semiconductor substrate in a method of forming asemiconductor.

In these cases, the drug solution or the vapor of the drug solution canbe a drug solution or vapor of a drag solution selected from the groupconsisting of nitric acid, sulfuric acid, ozonic water, hydrogenperoxide solution, mixed solution of hydrochloric acid and hydrogenperoxide solution, mixed solution of sulfuric acid and hydrogen peroxidesolution, mixed solution of aqueous ammonia and hydrogen peroxidesolution, mixed solution of sulfuric acid and nitric acid,nitrohydrochloric acid, perchloric acid, and boiling water. It ispreferable that the drug solution or the vapor of the drug solution canbe a drug solution or vapor of a drug solution selected from the groupconsisting of azeotropic nitric acid that is an azeotropic mixture ofnitric acid and water, azeotropic sulfuric acid that is an azeotropicmixture of sulfuric acid and water, and azeotropic perchloric acid thatis an azeotropic mixture of perchloric acid and water.

The film containing metal atoms can be a film containing metal atomsselected from the group consisting of aluminum, magnesium, nickel,chrome, platinum, palladium, tungsten, titanium, and tantalum.

The hydrogen-containing gas can be hydrogen or a mixed gas of hydrogenand a gas selected from the group consisting of nitrogen, argon, neon,water vapor, and oxygen.

Embodiment 2

The following describes a second embodiment of the present invention,with reference to FIGS. 8(a) to 12.

Described below with reference to FIGS. 8(a) to 8(e) is a method offorming a silicon dioxide film on a silicon substrate in accordance withthe present embodiment. In the present embodiment, the chemical oxidefilm 5 is formed on the silicon substrate 1 by the steps of FIGS. 8(a)to 8(c), which are respectively identical to FIGS. 1(a) to 1(c) ofEMBODIMENT 1. The description on the FIGS. 1(a) to 1(c) of EMBODIMENT 1is also applicable to the steps of FIGS. 8(a) to 8(c) of the presentembodiment.

After the chemical oxide film 5 was formed, in the present embodiment,the metal film 6 was not formed, but the wafer was heated in ahydrogen-containing gas in an electric furnace. In the presentembodiment, the wafer was heated in a mixed gas of hydrogen and nitrogen(nitrogen containing 5% of hydrogen) at 450° C. for 20 minutes.

This heat processing is performed to react hydrogen with interfacestates and defect states, and thereby cause them to disappear. In thisway, the electric property of the chemical oxide film 5 is improved.Since the metal film 6 is not formed in the present embodiment, thehydrogen-dissolving effect of the metal film 6 of EMBODIMENT 1 cannot beattained. However, the reaction of hydrogen with the interface statesand defect states can be accelerated by setting the temperature of thewafer in the heat processing higher than in the case of EMBODIMENT 1.The heat processing did not change the thickness of the chemical oxidefilm 5. As a result of the heat processing, a modified chemical oxidefilm 17 (hereinafter referred to as “modified oxide film” or “modifiedsilicon dioxide film”) was formed (FIG. 8(d)).

Examples of the hydrogen-containing gas used in the heat processing arehydrogen or a mixed gas of hydrogen and a gas selected from the groupconsisting of nitrogen, argon, neon, water vapor, and oxygen.

Therefore, other than in the mixed gas of hydrogen and nitrogen, theheat processing can also be performed, for example, in hydrogen, in amixed gas of hydrogen and nitrogen, in a mixed gas of hydrogen andargon, in a mixed gas of hydrogen and neon, in a mixed gas of hydrogenand water vapor, and in a mixed gas of hydrogen and oxygen.

It is not necessary that the temperature of the wafer in the heatprocessing is 450° C. Substantially the same effect of improving theelectric property of the chemical oxide film 5 is attained as long asthe temperature of the wafer is within the range of 300° C. to 600° C.Likewise, it is not necessary that the period of heat processing is 20minutes. Substantially the same effect of improving the electricproperty of the chemical oxide film 5 is attained as long as the periodof heat processing is 1 minute to 120 minutes. If the period of heatprocessing is not shorter than 1 minute, the hydrogen diffuses thechemical oxide film 5, thereby modifying the chemical oxide film 5effectively. In addition, if the period of heat processing is not longerthan 120 minutes, the increase of device preparation time is unlikely tobe a problem.

Thus, according to the present method, the chemical oxide film 5 can bemodified through the heat processing in the hydrogen-containing gas.Therefore, it is not necessary to perform heat processing in anoxidizing atmosphere. As a result, the thickness of the chemical oxidefilm does not increase easily, that is, excellent film-thicknesscontrollability is attained.

Furthermore, the heat processing in the hydrogen-containing gas iseffective even at a relatively low temperature. Therefore, the thicknessof the chemical oxide film does not increase easily even if oxygen orthe like is mixed in, that is, excellent film-thickness controllabilityis attained.

In order to increase the film thickness intentionally, thehydrogen-containing gas may be a mixed gas of hydrogen and water vapor,a mixed solution of hydrogen and oxygen, or the like.

Then, a metal film was formed on the modified chemical oxide film 17 andthe isolation region 2. Subsequently, a patterned resist film is formedon the metal film by a known photography technology. By a known dryetching technology, the metal film is etched and patterned to provide anelectrode 18. In this way, an MOS diode is formed (FIG. 8(e)).

Described next are properties of the modified chemical oxide film 17formed by the foregoing method. Unless otherwise noted, in the presentembodiment, the term “chemical oxide film formation processing” refersto the processing for forming the chemical oxide film 5 by soaking thewafer (finished with such processing as removal of natural oxide filmand cleaning) for 30 minutes in 68%-by-weight azeotropic nitric acidheated to the azeotropic temperature of 120.7° C. (processing of FIGS.8(a) to 8(c)), as in EMBODIMENT 1; and the term “oxide film modifyingheat processing” refers to the processing of introducing the wafer afterthe chemical oxide film formation processing into the electric furnaceand heating the wafer in nitrogen (which contains 5% of hydrogen) at450° C. for 20 minutes (processing of FIG. 8(d)).

Like FIG. 2 of EMBODIMENT 1, FIG. 9 is a graph illustrating therelationship between the applied voltage and the leak current density(measurement result) of the leak current flowing in the chemical oxidefilm. In FIG. 9, plot (a) is identical to the plot (a) in FIG. 2 ofEMBODIMENT 1, and indicates a leak current in the MOS diode formed byproviding the metal film 6 on the chemical oxide film 5 that was formedwithout performing the oxide film modifying heat processing. Plot (b)indicates a leak current of the MOS diode formed by performing oxidefilm modifying processing of the present embodiment after the chemicaloxide film formation processing.

From the comparison of the plot (a) and plot (b), it was found that theleak current density decreased drastically if the oxide film modifyingheat processing of the present embodiment was performed after thechemical oxide film formation processing. When the applied voltage was1V, the leak current density was about 0.5 A/cm². This current densitywas lower than that of the oxide film formed by high-temperatureoxidization.

FIG. 10 is a graph (C-V curve) illustrating the relationship between theapplied voltage and the electric capacitance of the MOS diode formed byperforming the oxide film modifying heat processing of the presentembodiment after the chemical oxide film formation processing. Like thecurve (b) in FIG. 3 of EMBODIMENT 1, the C-V curve of FIG. 10 did nothave a shoulder that appeared in the case where the chemical oxide film5 was formed without performing the oxide film modifying heatprocessing. This indicates that the defect states and interface statesdisappeared as a result of the oxide film modifying heat processing ofthe present embodiment.

FIG. 11 is a graph illustrating changes of the leak current density ofthe MOS diode formed by the chemical oxide film formation processing andoxide film modifying heat processing, the changes being caused bychanging the temperature of the wafer in the oxide film modifying heatprocessing. The result of FIG. 11 is about the leak current density ofthe case where the applied voltage is 1V, and indicates relative valuesof the leak current density (relative leak current density) on theassumption that the leak current density is 1 if the oxide filmmodifying heat processing is not performed.

From FIG. 11, it was found that the leak current density decreases notonly when the temperature of the wafer in the oxide film modifying heatprocessing was 450° C., but also when the temperature of the wafer waswithin the range of 300° C. to 600° C. It was also found that, if thetemperature of the wafer was within the range of 350° C. to 500° C., theelectric property of the modified chemical oxide film 17 improveddrastically, because the leak current density decreased to be not higherthan 50% of the case where the oxide film modifying heat processing wasnot performed.

If the temperature of the wafer in the oxide film modifying heatprocessing was 500° C. to 600° C., the defect states and interfacestates did not disappear effectively. As a result, the leak currentdecreased only by a small amount. On the other hand, if the temperatureof the wafer in the oxide film modifying heat processing was 350° C. to500° C., the defect states and interface states disappeared effectively.As a result, the leak current decreased drastically.

FIG. 12 is a C-V curve illustrating the relationship between the appliedvoltage and the electric capacitance of the MOS diode formed byperforming the oxide film modifying heat processing by heating the waferto a newly set temperature of 600° C. after the chemical oxide filmformation processing. Like the curve (a) in FIG. 3 of EMBODIMENT 1, theC-V curve of FIG. 12 had a shoulder that appeared in the case where thechemical oxide film 5 was formed without performing the oxide filmmodifying heat processing. This indicates that the defect states andinterface states do not disappear effectively through thehigh-temperature heat processing at 600° C.

As a result of experiments in which the oxide film modifying heatprocessing was performed by heating the wafer to various othertemperatures, a shoulder appeared on the C-V curve if the temperature ofthe wafer was not lower than 530° C. Therefore, it was found that thedefect states and interface states did not disappear effectively if thetemperature of the wafer was not lower than 530° C. These results showthat the oxide film modifying heat processing is required to beperformed at a temperature of not higher than 500° C.

In the method of patent publication 1 described in BACKGROUND ART, afterthe oxide film is formed, the oxide film is modified by heating in aninactive gas such as nitrogen. In this case, the interface statesdisappear when the stress on the interface of silicon/oxide film ismitigated through heat processing at a high temperature. Therefore, ingeneral, a high temperature of about 900° C. is required.

In contrast, according to the method of the present embodiment, theoxide film modifying heat processing is performed in an atmosphere ofhydrogen. This causes hydrogen to react with the interface states anddefect states and thereby form Si—H bonds. As a result, the interfacestates and defect states disappear. In this case, if the temperature ofthe oxide film modifying heat processing is lower than 350° C., it isdifficult to form the Si—H bonds by reacting the hydrogen with theinterface states and defect states. If, on the other hand, thetemperature of the oxide film modifying heat processing is higher than500° C., the Si-H bonds split up even if they are successfully formed,and the interface states and defect states are generated again.Therefore, in order to cause disappearance of the interface states anddefect states effectively, it is preferable to set the temperature ofthe oxide film modifying heat processing to 350° C. to 500° C.

FIG. 13 is a graph illustrating X-ray photoelectron spectrums from the2p orbit of silicon atoms of the chemical oxide film formed on thesilicon substrate 1. In FIG. 13, spectrum (a) is an X-ray photoelectronspectrum observed after the chemical oxide film 5 was formed by thechemical oxide film formation processing; spectrum (b) is an X-rayphotoelectron spectrum observed after (i) the chemical oxide filmformation processing was performed, and (ii) the oxide film modifyingheat processing was performed in a mixed gas of hydrogen and nitrogen(nitrogen that contains 5% of hydrogen) at 450° C. for 20 minutes.

The peak (1) in FIG. 13 is caused by the photoelectrons from the 2porbit of silicon atoms of the silicon substrate 1. The peak (2) in FIG.13 is caused by photoelectrons from the 2p orbit of the silicon atoms ofthe chemical oxide film 5 or the modified chemical oxide film 17. Basedon the ratio of the area intensity at the peak (2) with respect to thearea intensity at the peak (1), the thickness of the chemical oxide film5 and the modified chemical oxide film 17 was calculated. Thecalculation method was the same as the method employed in EMBODIMENT 1.

The thickness of the chemical oxide film 5 was calculated as 1.4 nm inthe case where the oxide film modifying heat processing was notperformed after the formation of the chemical oxide film (the casecorresponding to spectrum (a)). The thickness of the modified chemicaloxide film 17 was also calculated as 1.4 nm in the case where the oxidefilm modifying heat processing was performed after the formation of thechemical oxide film (the case corresponds to spectrum (b)). From thisexperimental result, it was found that the oxide film hardly grewthrough the oxide film modifying heat processing at 450° C. Thoughsimilar experiments, it was found that the thickness of the modifiedchemical oxide film 17 hardly increased even if the temperature of theoxide film modifying heat processing was changed within the temperaturerange of 300° C. to 600° C.

The extremely thin silicon dioxide film formed by the method of thepresent embodiment as described above can be used not only as anextremely thin gate oxide film of MOS transistors and MOS capacitors,but also for various other purposes. For example, it is possible toadopt the method of forming a silicon dioxide film in accordance withthe present embodiment as a step of forming a silicon dioxide film onthe surface of a silicon substrate in a method of manufacturing asemiconductor device.

Embodiment 3

The following describes a third embodiment of the present invention,with reference to FIG. 14.

In order to manufacture a flexible liquid crystal display, which hasbeen under development recently, it is necessary to form TFTs on asubstrate made of organic substance such as PET. For this purpose, theTFTs must be formed at a low temperature of not higher than 200° C.

In the case of such TFTs, in general, a relatively high voltage isapplied to the gate electrode. Therefore, a silicon dioxide film used asthe gate oxide film of the TFTs must have a sufficient thickness so asto avoid dielectric breakdown.

Conventionally, the silicon dioxide film used as the gate insulatingfilm of the TFTs has been formed through deposition by CVD. However, inorder to deposit the silicon dioxide film by CVD, the substrate must beheated to a high temperature of 400° C. to 500° C. Therefore, depositingthe silicon dioxide film by CVD is not suitable for forming the TFTs inmanufacturing a flexible liquid crystal display.

In order to form the oxide film at a low temperature of not higher than200° C., the method of forming a chemical oxide film by means ofexposure to vapor of a drug solution described in EMBODIMENT 1 can beemployed suitably.

Specifically, by exposing a PET substrate provided with a silicon filmon the surface thereof to vapor of a drug solution, and heating the PETsubstrate, the chemical oxide film (silicon dioxide film) can be formedon the surface of the silicon film.

The vapor of a drug solution can be a vapor of a drug solution selectedfrom the group consisting of nitric acid sulfuric acid, ozonic water(water in which several dozen ppm of ozone is dissolved), hydrogenperoxide solution, mixed solution of hydrochloric acid and hydrogenperoxide solution, mixed solution of sulfuric acid and hydrogen peroxidesolution, mixed solution of aqueous ammonia and hydrogen peroxidesolution, mixed solution of sulfuric acid and nitric acid,nitrohydrochloric acid, perchloric acid, and water.

Among these, it is preferable to use vapor of strong acid such as nitricacid, sulfuric acid, and perchloric acid. By using the vapor of strongacid, it is possible to oxidize the silicon film at a low temperature ofnot higher than 200° C., and thereby easily form a chemical oxide filmhaving a thickness of not less than 2 nm.

It is preferable that the vapor of a drug solution is vapor of a drugsolution selected from the group consisting of azeotropic nitric acidthat is an azeotropic mixture of nitric acid and water, azeotropicsulfuric acid that is an azeotropic mixture of sulfuric acid and water,and azeotropic perchloric acid that is an azeotropic mixture ofperchloric acid and water. In an azeotropic state, the concentration ofthe vapor of the drug solution does not easily change with time.Therefore, the thickness of the chemical oxide film to be formed can becontrolled with excellent reproducibility. This is why the vapor of theforegoing drug solutions is preferable.

It is preferable that the substrate temperature is within the range of50° C. to 200° C. when the PET substrate provided with the silicon filmon the surface thereof is exposed to the vapor of a drug solution. Ifthe substrate temperature is not lower than 50° C., oxidization isaccelerated effectively. Therefore, it is possible to effectively form achemical oxide film 5 having a thickness of not less than 1 nm (if thesubstrate temperature is lower than 50° C., it is difficult to form achemical oxide film 5 having a thickness of not less than 1 nm). Thesubstrate temperature is not higher than 200° C. in order to preventmodification and the like of the PET substrate.

The properties of the chemical oxide film formed by the foregoing methodare described below. In checking the properties of the chemical oxidefilm, it does not matter whether the chemical oxide film is a siliconfilm formed on a PET substrate or a silicon film formed on a siliconsubstrate. Therefore, in this case, a silicon substrate having a siliconfilm on its surface is used instead of the PET substrate having asilicon film on its surface.

FIG. 14 is a graph illustrating the X-ray photoelectron spectrum fromthe 2p orbit of silicon atoms measured after (1) the chemical oxide filmwas formed by heating the silicon substrate (cleaned as in EMBODIMENT 1)to 150° C., and (2) the silicon substrate was exposed to vapor ofazeotropic nitric acid for 30 minutes. The large intensity peak in FIG.14 is caused by the photoelectrons from the 2p orbit of silicon atoms ofthe chemical oxide film; the two weak peaks are caused by the siliconsubstrate.

Based on the ratio of the intensities at these peaks, the thickness ofthe chemical oxide film on the silicon substrate was calculated as 12nm. By thus using the vapor of a drug solution, it is possible to formthe chemical oxide film on the silicon substrate at a low temperature of150° C. In addition, by using the vapor of strong acid, it is possibleto oxidize the silicon film at a low temperature of 200° C. and therebyeasily form a chemical oxide film having a thickness of not less than 2nm, or even not less than 10 nm.

The method of forming a chemical oxide film in accordance with thepresent embodiment is particularly suitable for forming the gate oxidefilm of the TFTs of a flexible liquid crystal display. In addition, themethod is also suitable for forming a chemical oxide film that isrequired to be formed at a low temperature.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

The present invention relates to a method of forming a silicon dioxidefilm on a surface of a silicon substrate and a method of forming anoxide film on a surface of a semiconductor substrate, and a method ofmanufacturing a semiconductor device by these methods. These methods canbe employed to form an oxide film of an MOS (metal-oxide-semiconductor)device used for semiconductor integrated circuits, especially to form anextremely thin gate oxide film, a capacitor oxide film, and the like ofan MOS transistor, an MOS capacitor, and the like. In particular, thepresent invention relates to a method of forming a high-quality (withlittle leak current), extremely thin silicon dioxide film and the likewith excellent film-thickness controllability.

The present invention also relates to a method of forming, for example,a silicon dioxide film of a TFT (thin-film transistor) at a lowtemperature.

1. A method of forming an oxide film on a surface of a siliconsubstrate, comprising the steps of: forming an oxide film on a surfaceof a silicon substrate by supplying a drug solution or vapor of a drugsolution to the surface of the silicon substrate; depositing a filmcontaining metal atoms on the oxide film; and heat-processing thesilicon substrate, on which the film containing metal atoms isdeposited, the heat processing being conducted at a temperature of 100°C. to 250° C. in a hydrogen-containing gas.
 2. A method of forming anoxide film on a surface of a silicon substrate, comprising the steps of:forming an oxide film on a surface of a silicon substrate by supplying adrug solution or vapor of a drug solution to the surface of the siliconsubstrate; and heat-processing the silicon substrate, on which the oxidefilm is deposited, the heat processing being conducted at a temperatureof 350° C. to 500° C. in a hydrogen-containing gas, without depositing afilm containing metal atoms on the oxide film.
 3. The method as setforth in claim 1, wherein: the drug solution is selected from the groupconsisting of nitric acid, sulfuric acid, ozonic water, hydrogenperoxide solution, mixed solution of hydrochloric acid and hydrogenperoxide solution, mixed solution of sulfuric acid and hydrogen peroxidesolution, mixed solution of aqueous ammonia and hydrogen peroxidesolution, mixed solution of sulfuric acid and nitric acid,nitrohydrochloric acid, perchloric acid, and
 4. The method as set forthin claim 1, wherein: the drug solution is selected from the groupconsisting of azeotropic nitric acid that is an azeotropic mixture ofnitric acid and water, azeotropic sulfuric acid that is an azeotropicmixture of sulfuric acid and water, and azeotropic perchloric acid thatis an azeotropic mixture of perchloric acid and water.
 5. The method asset forth claim 1 wherein: the film containing metal atoms is a filmcontaining metal atoms selected from the group consisting of aluminum,magnesium, nickel, chrome, platinum, palladium, tungsten, titanium, andtantalum.
 6. The method as set forth claim 1, wherein: thehydrogen-containing gas is hydrogen or a mixed gas of hydrogen and a gasselected from the group consisting of nitrogen, argon, neon, watervapor, and oxygen.
 7. The method as set forth in claim 6, wherein: theheat processing in the hydrogen-containing gas is conducted for a periodwithin a range of 1 minute to 120 minutes.
 8. The method as set forth inclaim 1, wherein: a natural oxide film or impurities existing on thesurface of the silicon substrate are removed before forming the oxidefilm on the surface of the silicon substrate.
 9. The method as set forthclaim 1, wherein: the silicon substrate is heated in supplying the vaporof a drug solution to the surface of the silicon substrate.
 10. Themethod as set forth in claim 9, wherein: a temperature of the siliconsubstrate heated in supplying the vapor of a drug solution to thesurface of the silicon substrate is within a range of 50° C. to 500° C.11. A method of manufacturing a semiconductor device, comprising thestep of: forming an oxide film on a surface of a silicon substrate bythe method as set forth in claim
 1. 12. (canceled)
 13. (canceled) 14.(canceled)
 15. (canceled)
 16. (canceled)
 17. (canceled)
 18. (canceled)19. (canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled) 23.(canceled)
 24. The method as set forth in claim 2, wherein: the drugsolution is selected from the group consisting of nitric acid, sulfuricacid, ozonic water, hydrogen peroxide solution, mixed solution ofhydrochloric acid and hydrogen peroxide solution, mixed solution ofsulfuric acid and hydrogen peroxide solution, mixed solution of aqueousammonia and hydrogen peroxide solution, mixed solution of sulfuric acidand nitric acid, nitrohydrochloric acid, perchloric acid, and boilingwater.
 25. The method as set forth in claim 2, wherein: the drugsolution is selected from the group consisting of azeotropic nitric acidthat is an azeotropic mixture of nitric acid and water, azeotropicsulfuric acid that is an azeotropic mixture of sulfuric acid and water,and azeotropic perchloric acid that is an azeotropic mixture ofperchloric acid and water.
 26. The method as set forth in any one ofclaim 2, wherein: the film containing metal atoms is a film containingmetal atoms selected from the group consisting of aluminum, magnesium,nickel, chrome, platinum, palladium, tungsten, titanium, and tantalum.27. The method as set forth in claim 2, wherein: the hydrogen-containinggas is hydrogen or a mixed gas of hydrogen and a gas selected from thegroup consisting of nitrogen, argon, neon, water vapor, and oxygen. 28.The method as set forth in claim 27, wherein: the heat processing in thehydrogen-containing gas is conducted for a period within a range of 1minute to 120 minutes.
 29. The method as set forth in claim 2, wherein:a natural oxide film or impurities existing on the surface of thesilicon substrate are removed before forming the oxide film on thesurface of the silicon substrate.
 30. The method as set forth in claim2, wherein: the silicon substrate is heated in supplying the vapor of adrug solution to the surface of the silicon substrate.
 31. The method asset forth in claim 30, wherein: a temperature of the silicon substrateheated in supplying the vapor of a drug solution to the surface of thesilicon substrate is within a range of 50° C. to 500° C.
 32. A method ofmanufacturing a semiconductor device, comprising the step of: forming anoxide film on a surface of a silicon substrate by the method as setforth in claim 2.